For modern tramcars, due to their low-floor structure, the mounting space for a brake system is limited, thus a brake pressure is generally provided by a highly integrated valve-block type hydraulic unit.
FIG. 1 shows a composition diagram of a prior tramcar hydraulic brake unit, including an electrical element protection case 1, an integrated valve block 2, an oil tank 3, a vent plug 4, etc. Since the normal operation of a hydraulic pump and other elements inside the oil tank 3 can be ensured only if the oil tank 3 is communicated with the atmosphere smoothly, the vent plug 4 is generally mounted in a mounting hole on the upper surface of the oil tank 3. The mounting hole is a through-hole for ensuring the communication of the inner chamber of the oil tank with the atmosphere. The vent plug 4 is somewhat hydrophobic, and in general cases, can ensure the communication of oil liquid in the oil tank 3 with the atmosphere and prevent water vapor from getting into the oil tank 3.
In high-latitude areas, it is often rainy, snowy and frozen in the winter. Since the hydraulic unit is mounted on the bottom of a tramcar and very close to the ground surface, the surface of the vent plug 4 will be covered with snow, and the accumulated snow will be molten by heat generated during the operation of the elements inside the hydraulic unit, so that a lot of water will be accumulated on the surface of the vent plug 4. Consequently, the ventilation performance of the oil tank 3 will be influenced, or even the accumulated water will be suck into the oil tank 3 through the vent plug 4 under the action of a negative pressure, thereby influencing the operation of the hydraulic brake system and threatening the driving safety.
The prior art (CN20367070U) discloses a waterproof vent plug for a speed reducer and a speed reducer with the same, comprising a vent pipe, a connecting pipe and a cover, wherein the vent pipe is cup-shaped and has a peripheral wall and a bottom wall, on both of which at least one vent hole is provided to realize the ventilation of the inside of a housing and the outside, referring to FIG. 2. The vent plug can prevent water vapor from getting into the inside of the housing of the speed reducer, and avoid the degradation of lubrication effect caused by the increased content of water in the lubricating oil. However, the cover plate, the connecting pipe and the vent pipe are mounted on the vent plug structure successively from top to bottom, and the air flow path is of an unidirectional and longitudinal structure. Thus, when the surface of the oil tank is covered with ice or snow, it is still likely to result in the poor ventilation and the rain or snow penetration after the vent plug is covered with rain or snow, so that the driving safety of a train is affected.